The 100-, 300-, 500-, 700- and 900-mm soil layers at the grassland andE. viminalis sites studied, and the soil samples usedinthe sensor calibration, are described in terms of their particle size distribution, bulk density, electrical conductivity, soil pH,
saturation extract, Van-Genuchten parameters and porosity (Tables Al to A 7). The water retention characteristics (pressure head vs volumetric soil water content) of these soil layers are given in Figure Al. The water retention characteristics up to 1 bar was determined using a controlled outflow cell technique (Lorentz, 1993) and over 1 bar using the standard pressure plate technique (Dane and Hopmans, 2002). The Van- Genuchten parameters were determined with RETC, using the data from the water retention curves (Table A6).
The diagnostic soil horizons at the grassland andE. viminalis sites are described in tenns of their colour, structure, consistency, and occurrence of fragments and nodules (Tables A 8 to Al 0).
Table A.1 Particle size distribution, expressed as a percentage, for different soil depths at the grassland andE. viminalis sites studied, and for the soil samples used in the sensor calibration
Soil information Particle size distribution
Site Soil form Soil depth Clay Silt Sand
(mm)
(%) (%) (%)
Grassland Rensburg 100 24 24 49
300 46 16 34
500 56 14 30
700 50 17 31
E. viminalis Rensburg 100 36 17 43
300 41 16 38
500 50 13 34
700 36 16 47
900 37 15 46
E. viminalis Arcadia 100 26 20 52
300 44 15 38
500 48 11 39
Calibration Rensburg 100 21. 23 56
300 36 33 32
500 33 27 40
700 19 21 59
Table A.2 Bulk densities for different soil depths at the grassland andE. viminalis sites studied, and for the soil samples used in the sensor calibration
Soil depth (mm) Bulk density (kg m"j)
Site Grassland E. viminalis Calibration
Soil form Rensburg Rensburg Rensburg
100 1.603 1.556 1.402
300 1.526 1.383 1.401
500 1.363 1.385 1.273
700 Not available 1.360 1.519
Table A.3 Electrical conductivity for different soil depths at the grassland and E. viminalissites, for the Rensburg and Arcadia soil forms
Depth (mm) Electrical conductivity (mS mol) Site Grassland E. viminalis E. viminalis
Soil form Rensburg Rensburg Arcadia
100 44.20 27.90 34.10
300 41.60 32.90 25.00
500 86.50 56.00 80.30
700 94.70 35.10 Not available
900 Not available 41.20 Not available
Table AA Soil pH (H20) for different soil depths at the grassland andE. viminalis sites, for both the Rensburg and Arcadia soil forms
Depth (mm) Soil pH
Site Grassland E. viminalis E. viminalis
Soil form Rensburg Rensburg Arcadia
100 4.96 6.53 5.05
300 7.02 6.68 6042
500 7.87 8.25 8.12
700 8.04 8.15 Not available
900 Not available 8.13 Not available
Table A.5Na, Ca, MgandKsaturation extracts and sodium adsorption ratio (SAR) for different soil depths at the grassland andE. viminalissites for both the Rensburg and Arcadia soil fonns
Saturation extract (me L-1)
Site Soil form Depth NaZ7 Ca Mg K SAR
(mm)
Grass Rensburg 100 0.71 1.56 1.12 0.23 0.61
300 2.18 0.93 0.88 0.02 2.29
500 4.49 2.07 2.15 0.04 3.09
700 5.28 2.20 2.29 0.06 3.52
E. Arcadia 100 0.48 1.17 0.89 0.18 0.47
viminalis 300 0.99 0.67 0.56 0.03 1.26
500 4.36 1.85 1.58 0.02 3.33
E. Rensburg 100 1.51 0.55 0.49 0.03 2.09
vim in alis 300 1.90 0.54 0.49 0.02 2.65
500 3.35 1.15 1.13 0.03 3.14
700 1.88 0.87 0.85 0.03 2.03
900 2.91 0.58 0.57 0.05 3.84
27 Jcmol (+ or - charge) kg'l = 10 mmol (+ or - charge) kg'l= 1 me 100 g'l=10 me kg'l
Table A.6 Van-Genuchten parameters estimated with RETe (Van Genuchten et al., 1991) for different soil layers at theE. viminalis and grassland sites, and for the soil samples used in the sensor calibration
Site Soil Depth Van-Genuchten parameters
form (mm)
Bs BR a It m
(m3m-3) (m3m-3) (mm-I)
Grassland Rensburg 100 0.395 0 0.0088 1.0956 0.0872
300 0.427 0 0.0132 1.0813 0.0752
500 0.49 0 0.0086 1.114 0.1023
700
Fitted No No Yes
E. viminalis Rensburg 100 0.394 0 0.0276 1.0816 0.0754
300 0.478 0 0.029 1.0834 0.0769
500 0.482 0 0.0165 1.0346 0.0335
700 0.495 0 0.0032 1.0547 0.0519
Fitted No No Yes
Calibration Rensburg 100 0.35 0 0.0008 1.2075 0.1718
300 0.57 0 0.007 1.1028 0.0932
500 0.52 0 0.0002 1.3267 0.2462
700 0.44 0 0.0009 1.1632 0.1403
Fitted No No Yes
Table A. 7 Porosity for different soil depths at the grassland andE. viminalis sites, and for the soil samples used in the sensor calibration
Soil depth (mm) Porosity(m3m-3)
Site Grassland E. viminalis Calibration
Soil form Rensburg Rensburg Rensburg
100 0.395 0.413 0.471
300 0.424 0.478 0.471
500 0.486 0.477 0.520
700 Not available 0.487 0.427
0.6 0.5
11
Grassland site
0.3 004
Volumetric soil water content +100 mm 11300 mm "500 mm 0.2
1000 - - - . - - - = - - - , - - - , - - - 1 900
800 700 600 500 400 300 200
100 . . . " "
oL---r---r--~=---'·~___T__'..._----''---"__. . . , ; . . - - - _ _ _ _ j 0.1
E. viminalis site
1200~--.---~---___,---,---,
1000j --- -.-.- -- ---.--- ~_.. __ .. _. ._. • ._. ._._.L ._. c __ . . , . __ . _
600
•
x
" 'X +
400
200~---.--.-------,-- -- -- ---.- .... -. ,~__.L. .. __ _ .
•
800
+.
""• 100 mm 11 300 mm "500 mm x 700 mm Calibration data
160000,---~---.---_c_---,-,__---,
140000
•
0.50 0.60
0.30 0040
Volumetric soil water content +100 mm 11 300 mm .l> 500 mm x 700 mm 0.20
•
o- l - - - - - +-_-=--_~.=___~...!+L.....x...X><HO':''---'''----'''-><_+___;I*":;;I~~...::.l>~,,'---'''-___r____-_~_ ___j 0.10
20000
120000~- ---.- ---.- .c__ -- ._.__ __,__ _ _ .. .. _. ,_____ .__.__ ._._., ._. __
I
100000~
80000J
6000040000
Fig. A.I Top to bottom: Water retention functions (pressure head vs volumetric soil water content) for different soil depths at the grassland and E. viminalis sites, and for the soil samples used in the sensor calibration
AA Descriptionof soil profiles and diagnostic soil horizons at grassland andE. viminalis sites
Soils at the grassland andE. viminalis sites were classified according to the Taxonomic Soil Classification System for South Africa (Soil Classification Working Group, 1991).
Two soil forms were identified in the vicinity of the grassland andE. viminalis sites, namely the Rensburg and Arcadia soil forms. The Rensburg soil form was found at both the grassland andE. viminalis sites, whereas the Arcadia soil form was only found at theE. viminalis site. Du Toit (1993) noted that a broad range of soil forms (from Rensburg to Swartland) occurred within the Brandspruit Management Unit area, and included the Rensburg and Arcadia soil forms. Both the Rensburg and Arcadia soil forms consist of clays with an expansive nature. This could possibly hamper root development and would result in shallow effective soil depths. The subsoil of both the Rensburg and Arcadia soil forms are also generally prone to waterlogging during summer.
The Rensburg and Arcadia soil forms are described in terms of colour, structure and consistence (Tables A.8 to A.l 0) according to the Taxonomic Soil Classification System for South Africa (Soil Classification Working Group, 1991).
A.4.1 Description of the Rensburg soil profile at the grassland andE.
viminalis sites
The Rensburg soil form consists of a Vertic A-horizon (0 to 0.50 m) overlaying a G- horizon (deeper than 0.50 m). The Vertic A-horizon has a high clay content. The smectic clay minerals possess the capacity to swell and shrink markedly in response to soil water changes. The swell-shrink potential is manifested typically by the formation of vertical cracks in the dry state and the presence, at some depth, of slicken sides (Soil Classification Working Group, 1009). The Vertic A-horizons have a characteristic appearance with a strongly developed structure, ranging from moderate blocky to strong, medium angular blocky. The Vertic A-horizon had a very dark colour, ranging from black to very dark black clay. The dark colour develops under semi-arid to sub- humid climates. The parent material is either rocks that are basic or intermediate with
regards to the base reserve, or sediments in landscape positions which receive additions of bases via lateral drainage of water (Soil Classification Working Group, 1990). The A-horizon has a firm consistency when moist and has a moderate to strong medium angular blocky structure. The A-horizon at the grassland site had a strong medium angular structure, with a firm consistency (Table A.8). The A-horizon of the Rensburg soil form at theE. viminalis site had a moderate medium angular blocky to a weak fine blocky structure when dry (Table A.9).
Table A.8 Soil profile description of the Rensburg form at the grassland site Form: Rensburg
Locality: Secunda Site: Grassland
Family: Rietkuil Vegetation: Grassland
Soil Description Diagnostic
depth horizons
(mm)
100 Colour -Black (7.5YR/2.5/1) (moist) Vertic A Structure -Moderate blocky
Consistence -Firm
300 Colour -Very dark clay (7.5 YR/3/l) (moist) Structure -Strong medium angular blocky Consistence -Firm
500 Colour -Olive grey (5y/5/2) (moist) G Structure -Strong medium angular blocky
Fragments -Few (20 %) angular gravel fragments (less than 10 mm)
Consistence -firm
700 Colour -Olive grey (5 YR/5/2) (moist)
Fragments -Common angular gravel fragments Nodules -Few fme soft lime and manganese nodules Consistence - Firm
Table A.9 Soil profile description of the Rensburg fonn at theE. viminalissite Form: Rensburg
Locality: Secunda Site: Rensburg site
Family: Rietkuil
Vegetation: E. viminalis
Depth Description Diagnostic
(mm) Horizons
100 Colour - Black clay (5 YR/2.5/l) (moist) Vertic A Structure - Moderate medium angular blocky
(dry)
Consistence - Finn
300 Colour - Black clay (7.5YR/2.5/l) (slightly moist) Structure - Moderate medium angular (dry) Consistence - Finn
500 Colour - Dark grey (10 YR/4/l) (slightly moist) Structure - Moderate medium angular blocky (dry)
Consistence - Finn
Fragments - Few fine lime
700 Colour - Dark greyish brown (2.5Y/4/2) (slightly moist)
Fragments - Few manganese and lime fragments Structure - Weak fine blocky (dry)
Consistence - Slightly finn
Fragments - Few fine manganese and lime fragments
900 Colour - Dark brown (IOYR/3/3) Weathering
Weathering dolorite dolorite
The G-horizon (gleyed) is generally saturated with water for long periods, unless drained. The G-horizon has an olive grey colour (grass) or a colour ranging from dark grey through to dark greyish brown. Gleying, with the reduction of ferric oxides and hydrated oxides, is the essential process to which the G-horizon is subjected. Grey, blue and green colours predominate, but stains of ferric and manganese oxides and hydrates (yellow, brown, red and black) may be present and indicate localized areas of better aeration. Grey colours are due to an absence of iron compounds, and blue and green are due to the presence of ferrous compounds.
The G-horizon at the grassland site had a slightly firm to firm consistency when wet.
The structure of the G-horizon ranged from strong medium angular (500 mm) to weak fine blocky (700 mm). Angular fragments occurred, becoming common with depth. A few fine soft lime and (iron-) manganese nodules also occurred. A few fine manganese and lime fragments occurred. Lime and manganese fragments suggest higher levels of alkalinity as a result of reduced leaching in the G-horizon in this horizon compared to the Vertic A-horizon. (Usually, but not always, marked clay illuviation takes place especially in the upper part of the G-horizon) (Table A.8).
A.4.2 Description of the Arcadia soil profile at the E. viminalis site
The Arcadia soil form (Table A.lO) consists of a Vertic A-horizon (0 to 700 mm), overlaying parent material (e.g. sandstone). The Vertic A-horizon has a black to black clay colour close to the soil surface. A very dark greyish brown layer overlies the weathering sand stone. The structure of this horizon varies between weak medium crumb (100 mm) and strong medium blocky when dry and the consistency ranged between slightly firm (300 mm) to firm when moist. Very hard, fine lime nodules occurred at depths exceeding 500 mm (Table A.lO).
Table A.l 0 Soil profile description ofthe Arcadia fonn at the tree site Form: Arcadia
Locality: Secunda Site: Arcadia site
Family: Rustenburg Vegetation: E. viminalis
Soil Description Diagnostic
depth Horizons
(mm)
100 Colour - Black (7.5YRJ2.5/1) (slightly moist) Vertic A Structure - Weak medium crumb (dry)
Consistence - Slightly finn (moist)
300 Colour - Black clay (7.5YRJ2.5/l) (slightly moist) Structure - Strong medium blocky (dry)
Consistence - Finn (moist)
500 Colour - Very dark greyish brown(2.5YRJ3/2) Lime nodules (high pH) - Very hard fine lime nodules
Structure - Moderate fine blocky (dry) Consistence - Finn (moist)
700 Sandstone Sandstone
APPENDIX B
LABORATORY CALIBRATION OF SOIL SENSORS FOR SITE-SPECIFIC CONDITIONS
B.1 Introduction
As part of the field experiment, soil water content and soil water potential was estimated at different depths. These sensors were not calibrated prior to or during the field
experiment. However, relationships between sensor output and soil water content and soil water potential is required to convert sensor output to soil water content (water content reflectometer) or to soil water potential (heat dissipation sensor and
thermocouple psychrometer). Some sensors and their calibration relationships are sensitive to site specific conditions and application of these calibration equations to non-standard conditions could lead to inaccurate estimations of soil water content or soil water potential. For example a water content reflectometer is sensitive to the electrical conductivity, clay and organic matter content and to air temperature.
Therefore, when this sensor is used under non-standard conditions (e.g. clay content greater than 30 %), the standard calibration polynomials may no longer apply (Campbell Scientific Inc., 1996). Therefore for use ofthe soil reflectometer sensor under non-standard conditions, individual calibration is required.
Therefore, soil water content (water content reflectometer) and soil water potential sensors (heat dissipation) used in the field experiment, were calibrated for field specific conditions. The thermocouple psychrometers were not calibrated in the laboratory since the heat dissipation calibration function was to be used to obtain a calibration function for the thermocouple psychrometer that relates sensor output to the field matric potential.
In addition to the water content reflectomers and the heat dissipation sensors, time domain reflectometers were included in the calibration. The reason for this was that these sensors are often used inresearch, and the question arose as to whether the equation suggested by Toppet al. (1980) and Ledieu et al. (1986) (cited by Campbell
Scientific Inc., 1992), for the calculation of the soil water content, would apply to soils with higher clay contents (greater than 30 %).
B.2 Materials and methods
Block soil samples of dimensions 500 mm x 300 mm x 200 mm were removed from the grassland site at the end of the field experiment. Block samples were only taken at the grassland site, as the soil properties at the grassland andE. viminalissites were similar and the clay contents within each layer were within 5 to 14 % of each other
(Appendix A).
The sides of an already opened trench at the grassland site, with a depth of approximately I m, were shaven back. A V-shaped metal frame was subsequently carefully hammered vertically into the soil (Fig. B.I). Once the complete height (200 mm) ofthe frame was in the soil, the block soil sample was cut loose from the rest ofthe soil. A thick metal plate was hammered into the soil horizontally, below theV- shaped metal frame. Care was taken so as not to disturb the block sample and
surrounding soil too much whilst removing the sample. Once the sample was cut loose from the bulk soil and removed from the trench, lids were used to cover the front, bottom and top of the soil sample. These covers kept the soil sample intact during transportation. Soil samples were taken at soil depths of 0 to 200 mm, 200 to 400 mm, 400 to 600 mm and 600 to 800 mm.
Fig. B.I Anexample of one of the V-shaped metal frames usedinthe sampling of block soil samples at the grassland site, for the water content reflectomer calibration
The four soil samples were then transported from the research site in Secunda to a laboratory in Pietermaritzburg, approximately 500 kmfrom the research site. In the laboratory, the front, top and bottom lids were removed, and each sample was carefully and fairly tightly wrapped first in mesh wire and then in canvas. The mesh wire and canvas kept the block soil sample intact, whilst wetting and prevented extensive swelling or expansion of the sample. The block samples were placed in a large
container (about 50t) half-filled with water, and allowed to saturate over a period of 14 days. The samples were not submerged in the water. The block samples were then removed and allowed to drain naturally.
Sets of soil sensors consisting of a water content reflectometer, heat dissipation sensor and a time domain reflectometer were then installed horizontally through the front of each block sample (Fig. B.2). A 300-mm long drill bit together with an installation guide was used to install the water content reflectometers and time domain reflectometers. A shorter drill bit was used to drill holes to install the heat dissipation sensors. Care was taken to ensure that the reflectometer rods were installed parallel to each other, and that all the sensors made good contact with the surrounding soil.
The soil sensors were subsequently connected to dataloggers. The water content reflectometers were connected to a Campbell CR23X logger, the time domain reflectometers to a CRI OX datalogger, and the heat dissipation sensors to a CR7X datalogger (Fig. B.2). The water content reflectometers and heat dissipation sensors were connected to the loggers exactly as was done during the field experiment. The same datalogger programmes that were used during the field experiment were also used during the calibrations. All sensor outputs (output period, propagation velocity and temperature and change in temperature for the water content reflectometer, time domain reflectometer and heat dissipation sensors respectively) were measured every four hours.
Fig. B.2 Horizontal installation of water content reflectometers, time domain
reflectometers and heat dissipation sensors (top) in the block soil samples used in the sensor calibration, and the connections of the sensors to CROlX, CR23X and CR7 dataloggers (bottom)
The continuous four-hourly measurements of the sensors outputs were combined with frequent measurements of the block sample weights. The block sample weights were measured using a digital scale with a resolution of 109. Measurements were obtained at similar times of day, and preferably in the morning. The calibration process lasted for approximately 170 days, until signs of cracks on the outer surfaces of the block samples were visible.
At the end of this calibration period, sub-soil samples were taken from each block sample. These sub-samples were analysed in terms of the water retention characteristics and physical properties. Sub-soil samples were also used to determine the final soil water content and oven dry weight of the sub-sample. The final soil water content of each sub-sample was assumed to be the same as that of the block samples, and was used to determine the gravimetric and volumetric soil water contents of each block sample throughout the calibration process.
B.3 Results and discussion
The relationship between the sensor output for each block sample and the volumetric soil water content estimated for the block sample was obtained. Second-order
polynomials, as used in the factory calibrations, were fitted between the water content reflectometer period (ms) and the measured volumetric soil water content of the block sample. However, linear relationships were fitted between the inverse of the
propagation velocity for the time domain reflectometer and the measured soil water content. Only the results of the water content reflectometer calibration functions will be discussed in this section. The results of the time domain reflectometer calibrations are not discussed, as it does not apply directly to the field experiment.
The actual volumetric soil water measured during the calibration process (Fig. B.3) shows the range of water contents under which the soil sensors were calibrated. The soil water content of the block samples at the beginning of the calibration period (26, 46, 52 and 39%respectively at the 100-,300-,500- and 700-mm soil depths) was within 9 % of the saturated soil water contents (Table B.l) (based on the sub-soil samples). This indicates that all four block samples were close to saturation at the start of the calibration.
According to the relative saturation (Eq. 3.8), (using saturated and residual soil water contents derived for sub-soil samples) and the block sample soil water contents, the water content reflectometers were calibrated over a wide range of soil wetness
(Fig. BA). The soil wetness ranged between 14 and 73 %,26 and 81 %,42 and 100 % and 33 and 75 % at the 100 mm28, 300 mm, 500 mm and 700 mm soil samples
respectively. The relationship between the sensor output and measured soil water content would therefore apply to both wet and dry soil conditions encountered during the field experiment.
28
The depths 100 mm, 300 mm, 500 mm and 700 mm refer to the depth at which the block soil samples were taken, and represents the soil layers: 0 to 200 mm, 200 to 400 mm, 400 to 600 mm and 600 to 800 mm respectively.
x 100 mm • 300 mm u 500 mm A700 mm 06
'"
E i..J
1-
0.5CQ)
•
Den DC ~
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•
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$ ~ A
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o 0ro 0
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06-Apr 17-Mar
26-Feb 07-Dec 27-Dec 17-Jan 06-Feb
Date (2001-2002) 17-Nov
0.0 +-_ _---,- -,--_ _---,- -,--_ _----r- ..,....-_ _----r- ..,....----l
28-0ct
Fig. B.3 Daily measured volumetric soil water content (mass based) for four soil samples used in the sensor calibration, over a period of 170 days. 100, 300, 500 and 700 mm represent the depths at which the calibration soil samples were taken.
Table B.l Van Genuchten water retention parameters (Eq. 3.4) for the calibration soil samples taken from different depths below the soil surface (100 to 700 mm)
Calibration data
Van Genuchten water retention parameters estimated with RETC29
Depth (mm) Bs BR a n m
100 0.35 0 0.00076 1.20745 0.17181
300 0.57 0 0.00701 1.10275 0.09318
500 0.52 0 0.00017 1.32669 0.24624
700 0.44 0 0.00087 1.16324 0.14033
29RETC is a programme for quantifying the hydraulic functions of a unsaturated soil. RETC was developed by Van Genuchten et al. (199 J).